In order to study the influence of sociodemographic factors, postneonatal mortality of all live births surviving the neonatal period registered in the Norwegian Medical Birth Registry in 1978-1982 were examined (n = 209,030). Postneonatal deaths (n = 634) were divided into two categories; deaths due to the sudden infant death syndrome (SIDS) (n = 359) and deaths due to other causes (non-SIDS) (n = 275). SIDS and non-SIDS deaths showed different relationships to sociodemographic factors, and the associations appeared to be different for first-born and later born children. SIDS mortality was highest for first-born offspring when the mother was young (adjusted relative risks (RR) 2.3) and had a low educational level (adjusted RR 4.9). For later-born offspring no association between maternal educational level and SIDS was found, while young maternal age (adjusted RR 4.4) and unmarried status (adjusted RR 2.3) were closely associated with SIDS. In the multivariate model, however, there were no statistically significant associations between non-SIDS and sociodemographic factors for first-born or later-born children. Thus it appears that the increased postneonatal mortality in lower social groups can be explained by an association with SIDS.